This invention relates generally to rotary power tools and specifically to means for attaching rotary work wheels thereto.
Presently, saw blades and grinding wheels, hereinafter referred to collectively as work wheels, are secured to the tool arbor or shaft by means of a washer and a suitable fastener. If the arbor has an external thread, a nut may be used as a fastener and if the arbor has an internal thread, a bolt may be used. In some instances in bench mounted grinders, for example, a thick, internally threaded washer with two holes in the washer face, for accommodating a spanner wrench, is used in place of a nut. In either case, a wrench is needed to tighten the threaded fastener to the threaded arbor or to remove the threaded fastener from the arbor. The arbor is rotatable and coupled to the drive gearing of the tool and, since most rotary tools are relatively free-turning, it is generally necessary to use two wrenches in counter rotation to tighten or loosen the threaded fastener. The second wrench must be placed over a hex drive head on the arbor flange, into a hole in the arbor itself or into holes in the arbor washer, all of which are located behind the work wheel. Consequently, the work wheel totally blocks visibility, and it is very difficult to position the second wrench. When the second wrench is properly installed, the operator must push on one wrench and pull on the other while his hands straddle the sharp or highly abrasive work wheel. Dislodgement of either wrench can result in injury to the operator from the sharp or abrasive work wheel.
Most rotary power tools are designed to self-tighten when in use, i.e. when the work wheel engages the workpiece, the resisting force acts to further tighten the threaded fastener. Under a heavy workload, the threaded fastener can become extremely tight and make its removal even more difficult and strenuous for the operator. The difficulty and potential for injury involved has resulted in machinists evolving their own methods of changing work wheels. One is to position a spanner wrench on the arbor nut and manually accelerate the wheel to drive the wrench handle into the wheel guard to loosen the threaded fastener. A series of such impacts may loosen the threaded fastener. Another method is to bang the end of the positioned spanner wrench with a hammer. While both methods are relatively simple and generally effective to loosen the threaded fastener, they can adversely affect the rotary tool. In a surface grinder, for example, damage may occur to the highly precisioned arbor bearings by the shock loads transmitted to them, which could materially reduce bearing life. Also, the sudden shock loads are very destructive to the so-called "live coupling" that transfers the rotary motion from the tool motor to the arbor.
The situation with respect to portable power tools, such as rotary power saws, is not much better because there is no ready access to the rear side of the blade, which is fitted with a keyed hole that matches a flange piece mounted to the shaft. The blade itself must therefore be held secure while a wrench or special tool is used to tighten the nut. Because of the keyed arrangement, restraining the blade prevents rotation of the saw gear train and motor shaft and permits the fastener to be tightened or loosened. Many haphazard and often dangerous techniques are used to hold the blade. A long, relatively thin object, such as a screwdriver, is often used to wedge the toothed blade against the saw housing to prevent movement during tightening and loosening. The object may be inserted through a hole (if available) in the side of the blade, or more usually wedged between a pair of saw teeth. While this technique could also have adverse effects on the saw, it is preferable to holding the sharp saw blade with one's hand.
The prior art has developed a number of effective techniques for removing threaded fasteners from arbors. Some commercial saws include a button-operated device that internally stops the arbor from rotating and eliminates the need for a second wrench. This system is built into the power tool and, of course, adds to the tool's complexity and cost. A fastening device similar to that of the present invention is shown in U.S. Pat. No. 2,704,426. That device uses a pinion gear to drive a spur gear that in turn threadingly engages the arbor. The spur gear loads a bearing plate that is keyed to the arbor and includes a hole for locating the end of the pinion gear adjacent to the spur gear.
The patented device has a short extension shaft on the end of the pinion gear that cooperates with the hole in the bearing plate to position and retain the pinion gear and spur gear in engagement. Operational ease requires some clearance or "play" between the extension shaft and the hole and between the teeth of the pinion gear and spur gear, which makes for an awkward arrangement. The two meshing gears tend to push apart when loaded. Because the pinion gear is retained at one end only, a cocking condition occurs that results in binding and excessive wear. The need for the extension shaft also severely restricts the minimum size of the device. Downsizing the patented device to fit hand power saws, for example, would result in an extremely fragile extension shaft. A thick bearing plate is required to provide a deep enough hole for the extension shaft to bear in. This adds to the overall thickness of the device which creates a clearance problem on many circular saws and hand-held power tools.
With the construction of the invention, however, the spur gear and the entire pinion gear are captivated in counter bores in the bearing plate. The pinion gear teeth actually bear against the walls of the cutout. The resulting novel axle-less gear drive permits very reliable operation without the need for close tolerances, and can be used in very small devices.
Thus, with the present invention, the problem of replacing work wheels on rotary tools is overcome in a very effective manner. Not only is the system of the invention simple, compact and inexpensive, it captivates the axle-less drive pinion gear during operation. Further the arrangement of the drive pinion gear and the spur gear in a recessed flange plate is not only compact, but enables smooth, controlled application of torque. The inventive construction is therefore eminently suited for use with portable rotary power tools.